1. Field of the Invention
The present invention relates to colloidal metal oxides with blocked isocyanate groups and their use for producing scratch-resistant coatings on any substrate.
2. Description of the Prior Art
There is a large demand for scratch-resistant coatings for sensitive surfaces, in particular thermoplastic materials. These materials are characterized by high impact strength, transparency and dimensional stability, but only have very low resistance to scratching and to chemicals.
Thermally curing silicone lacquers, which preferably crosslink by a condensation reaction to give Si--O--Si linkages, are mainly used to improve the scratch resistance of thermoplastic materials. In addition to this type of crosslinking, other crosslinking mechanisms may also be used. These coating compositions are often based on partial hydrolysis-condensation products of alkyl or aryltrialkoxysilanes which are usually combined with colloidal silica sols. These coating compositions are described, for example, in U.S. Pat. Nos. 3,790,527, 3,887,514, 4,243,720, 4,278,804, 4,419,405, 4,624,870, 4,680,232, 4,006,271, 4,476,281, in DE-A 4,011,045, 4,122,743, 4,020,316, 3,917,535, 3,706,714, 3,407,087, 3,836,815, 2,914,427, 3,135,241, 3,134,777, 3,100,532, 3,151,350 and in DE-A 3,005,541, 3,014,411, 2,834,606, 2,947,879, 3,016,021.
These coatings often have only low elasticity which means they can only be applied in thin layers of less than 10 .mu.m. In addition, due to the small proportion of organic compounds, they are incompatible with organic binders and have poor adhesion, which in most cases requires the application of a primer. In addition, a long baking time is often required for the condensation crosslinking reaction, which may take several hours depending upon the thermal sensitivity of the substrate.
An object of the present invention is to provide an elastic, scratch resistant coating composition, which can be applied in thick layers and has good adhesion to the substrate without the use of a primer. It is an additional object of the present invention to obtain these properties without having to accept a deterioration in other properties, such as scratch resistance and chemical resistance.
Surprisingly, these objects can be achieved with the colloidal metal oxides having blocked isocyanate groups described hereinafter. These compositions can be used to provide hard, scratch resistant coatings having a high resistance to chemicals and outstanding adhesion to the substrate without a primer. The compositions may be applied using conventional baking techniques for blocked isocyanates.